one element is missing while using Set( generator ) [closed]

7016 ●9 ●53 ●166

In the code:

import itertools
Q = [[factor(5),factor(13)],[factor(9),factor(7),factor(13)]]
print( sorted( Set( [lcm(*qq) for qq in itertools.product(*Q)] ) ) )
print( sorted( Set( (lcm(*qq) for qq in itertools.product(*Q)) ) ) )

first print produces the expected result:

[13, 5 * 7, 3^2 * 5, 5 * 13, 7 * 13, 3^2 * 13]

while second print produces a list with one element (3^2 * 5) missing:

[13, 5 * 7, 5 * 13, 7 * 13, 3^2 * 13]

Is this a bug?

ADDED. More weirdness - the following code

Q = [[factor(3), factor(9)], [factor(9), factor(7)]]
print( Set( [lcm(*qq) for qq in itertools.product(*Q)] ) )

produces a set with two equal elements:

Set of elements of [3^2, 3 * 7, 3^2, 3^2 * 7]

It seems that these issues are related to the fact that Factorization objects are not hashable, but why then does Set allow to create sets of them with all kinds of side effects?

Closed for the following reason question is not relevant or outdated by Max Alekseyev
close date 2024-02-19 18:39:24.561147

Comments

and Set is not working correctly with iterators

FrédéricC ( 3 years ago )

I can't reproduce the first issue: I get the same answer — the first one — when I repeat the print command. What version of Sage are you using? I agree with the final question: Set allows nonhashable objects, and it doesn't look very good, for example Set([[3,4], [3,4]]).

John Palmieri ( 3 years ago )

I use Sage 9.5 The first issue can be also seen in SageMathCell: https://sagecell.sagemath.org/?q=ainesk

Max Alekseyev ( 3 years ago )

Maybe it's been fixed in the 9.6 prerelease.

John Palmieri ( 3 years ago )

I can reproduce with 9.6.rc3 on macOS 12.3.1

David Coudert ( 3 years ago )

see more comments

diff --git a/src/sage/sets/set.py b/src/sage/sets/set.py index 96e3b2ad7a..ff133a315e 100644 --- a/src/sage/sets/set.py +++ b/src/sage/sets/set.py @@ -198,11 +198,12 @@ def Set(X=None): raise TypeError("Element has no defined underlying set") try: - X = frozenset(X) + Y = list(X) + Y = frozenset(Y) except TypeError: - return Set_object(X) + return Set_object(Y) else: - return Set_object_enumerated(X) + return Set_object_enumerated(Y) class Set_base():

Comments

Would there be an unneeded overhead for hashavle elements? Given various side effects, I'd rather disable Sets for unhashable elements.

Max Alekseyev ( 3 years ago )

After the initial construction (this try ... except block), a Set with hashable elements will be an instance of a different Python class than one with unhashable elements, so I hope there is no overhead.

John Palmieri ( 3 years ago )

Creating a list before creating a frozenset brings an overhead. Think about an object with millions of elements.

Max Alekseyev ( 3 years ago )

As far as I can tell, there is no way to access an iterator without modifying it, and frozenset(X) accesses it. I don't know another way to keep the current functionality and also to fix this bug. Users can explicitly import and call Set_object_enumerated on frozenset(X) if they know that will work.

John Palmieri ( 3 years ago )

Should I open a ticket for this issue?

Max Alekseyev ( 2 years ago )

see more comments

one element is missing while using Set( generator ) [closed]

Closed for the following reason question is not relevant or outdated by Max Alekseyev
close date 2024-02-19 18:39:24.561147

Comments

1 Answer

Comments

Question Tools

Stats

one element is missing while using Set( generator ) [closed] savecancel

Closed for the following reason question is not relevant or outdated by Max Alekseyev close date 2024-02-19 18:39:24.561147

Comments

1 Answer

Comments

Question Tools

Stats

one element is missing while using Set( generator ) [closed]

Closed for the following reason question is not relevant or outdated by Max Alekseyev
close date 2024-02-19 18:39:24.561147